On Some Common Errors in Iron Bridge Design

26 We now proceed to cite cases of infraction of these rules, with consequent loss of strength. Figure 22 represents one girder of a small over bridge at the Spencer Street Railway Station, Melbourne. It is 45 feet span and about 4 feet deep. All the diagonal web members are of 3 x inch iron. These are suitable enough for tension purposes, but most inefficient in compression, having a radius of gyration across the plane of the girder of only -1 inch or about of the length. This unfortunate girder further presents the faults of redundancy, owing to the presence of unnecessary verticals, and of absurdly heavy and complicated end pillars. Fig. 23 represents an. amended design in which the more heavily stressed compression diagonals are made of angle iron having a radius of gyration, many times greater than that of the flat bars, and in which the faults of redundancy and dispropor- tionate ends are avoided. Inch to the foot models in iron of each of these girders were constructed at the University and tested to destruction, with the following results:—1 he amended design, contained 16 per cent, less iron than the original owing to the omission of the massive end plates and intermediate verticals. It involved much less workmanship owing to there being less than one-third the number of rivets, and the time taken in making it was less than half that of the other. Its actual breaking load distributed along the bottom chord as in the actual bridge was 77libs, as against 208 for the original structure. Thus with identical external dimensions, and very little change in appearance,'the cost of the structure was largely reduced, and its strength increased nearly fourfold. The mode of fracture of this defective design was, as was predicted by calculation, the buckling or side ways bending of the weak compression diagonals near the end. There would be no difficulty and but little expense in increasing the strength of this structure threefold by simply clamping angle irons to the most heavily stressed compression diagonals so as to prevent their bending, and this ought to be done, and was long ago urged by the writer, but without result. lig. 24 represents the section of a large buttress or inclined strut, erected at great cost on the down stream side of the Victoria Street Bridge, near Melbourne, for the purpose of